U.S. patent application number 15/003348 was filed with the patent office on 2016-07-21 for self-heating fuel cell systems.
The applicant listed for this patent is EC Power, LLC. Invention is credited to Chao-Yang WANG.
Application Number | 20160211535 15/003348 |
Document ID | / |
Family ID | 56408497 |
Filed Date | 2016-07-21 |
United States Patent
Application |
20160211535 |
Kind Code |
A1 |
WANG; Chao-Yang |
July 21, 2016 |
SELF-HEATING FUEL CELL SYSTEMS
Abstract
Fuel cell systems, e.g. systems including proton exchange
membrane (PEM) fuel cells, are engineered to have more than one
internal electrical resistance that can change according to
temperature. Such changes in internal electrical resistance levels
allow rapid heat-up of the fuel cells from low temperatures to an
elevated temperature that is optimal for water management and fuel
cell operation. The fuel cell systems can include at least one fuel
cell and at least one resistor-switch unit electrically connected
to the at least one fuel cell. The at least one resistor-switch
unit includes a resistor and a switch in which the switch is
electrically connected in parallel with the resistor.
Inventors: |
WANG; Chao-Yang; (State
College, PA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
EC Power, LLC |
State College |
PA |
US |
|
|
Family ID: |
56408497 |
Appl. No.: |
15/003348 |
Filed: |
January 21, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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62105875 |
Jan 21, 2015 |
|
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M 2250/20 20130101;
Y02E 60/50 20130101; H01M 8/04037 20130101; H01M 8/04268 20130101;
H01M 8/04365 20130101; Y02B 90/10 20130101; H01M 8/04225 20160201;
H01M 2008/1095 20130101; H01M 2250/10 20130101; Y02T 90/40
20130101; Y02E 60/10 20130101 |
International
Class: |
H01M 8/04007 20060101
H01M008/04007; H01M 8/1018 20060101 H01M008/1018 |
Claims
1. A self-heating fuel cell system comprising at least one fuel
cell and at least one resistor-switch unit electrically connected
to the at least one fuel cell, wherein the at least one
resistor-switch unit comprises a resistor and a switch and wherein
the switch is electrically connected in parallel with the
resistor.
2. The system according to claim 1, comprising a first internal
resistance (R.sub.1) of the system when the temperature of the at
least one fuel cell is greater than a temperature (T.sub.1) and a
second internal resistance (R.sub.2) of the system when the
temperature of the at least one fuel cell is below T.sub.1, wherein
the value of R.sub.2 at about 2.degree. C. below T.sub.1 is at
least twice the value of R.sub.1 at T.sub.1.
3. The system according to claim 2, wherein T.sub.1 is less than
95.degree. C.
4. The system according to claim 1, wherein the fuel cell comprises
a bipolar plate with a flowfield for fuel flow on an anode, a
diffusion media on a membrane-electrode assembly, a bipolar plate
with a flowfield for oxidant flow on a cathode, wherein the
resistor is placed in direct contact with one of the bipolar
plates.
5. The system according to claim 4, wherein the resistor is inside
the at least one fuel cell, or outside of the at least one fuel
cell.
6. The system according to claim 1, comprising a stack of fuel
cells and wherein the at least one fuel cell is among the stack of
fuel cells and the resistor-switch unit is interposed between two
fuel cells in the stack of fuel cells.
7. The system according to claim 6, comprising multiple
resistor-switch units inserted between fuel cells over several
locations in the fuel cell stack.
8. The system according to claim 4, comprising the at least one
resistor-switch unit inserted between the diffusion media and one
of the bipolar plates of the at least one fuel cell.
9. The system according to claim 1, wherein the resistor comprises
an electrically conductive sheet made of graphite, stainless steel,
nickel, chrome, nichrome, copper, aluminum, titanium, or alloys
thereof.
10. The system according to claim 9, wherein the resistor is a thin
metal sheet having one or more electrically insulating films on one
or both major surfaces of the sheet.
11. The system according to claim 10, wherein the resistor has an
overall thickness between 1 and 100 micrometers.
12. The system according to claim 10, wherein the electrically
insulating films comprise polyethylene, polypropylene, chlorinated
polypropylene, polyester, polyimide, PVDF, PTFE, nylon, or
co-polymers thereof.
13. The system according to claim 1, further comprising a
controller configured to operate the switch to an open state or a
closed state based upon an input from a temperature sensing device
that can measure a surface temperature of the at least one fuel
cell.
14. The system according to claim 7, wherein the multiple switches
can be activated sequentially according to a control algorithm.
15. The system according to claim 1, wherein the at least one fuel
cell is a proton exchange membrane fuel cell that can operate on
hydrogen or hydrogen-containing gases.
16. The system according to claim 1, further comprising a DC/DC
converter electrically connected to the fuel cell, an inverter
electrically connected to the DC/DC converted, an electric motor
electrically connected to the inverter and a controller
electrically connected to the switch and configured open the switch
to generated heat in the resistor to heat the fuel cell.
17. A method of operating a self-heating fuel cell system, the
method comprising operating the system at a first internal
resistance (R.sub.1) when the temperature of at least one fuel cell
in the system is greater than a temperature (T.sub.1), and
operating the system at a second internal resistance (R.sub.2) of
the system when the temperature of the at least one fuel cell is
below T.sub.1, by activating a switch that activates either R.sub.1
or R.sub.2 depending on T.sub.1.
18. The method according to claim 17, wherein the at least one fuel
cell comprises a membrane-electrode assembly; an anode catalyst
electrode; a cathode catalyst electrode; a resistor-switch unit
comprising a resistor and switch, wherein the switch is
electrically connected in parallel with the resistor; a temperature
sensing device on the fuel cell; and a controller configured to
receive inputs from the temperature sensing device and to operate
the switch.
19. The method according to claim 18, wherein the controller opens
the switch on the resistor-switch unit based on an input from the
temperature sensing device.
20. The method according to claim 17, wherein T.sub.1 is less than
95.degree. C.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims the benefit of U.S. Provisional
Application No. 62/105,875 filed Jan. 21, 2015 the entire
disclosure of which is hereby incorporated by reference herein.
TECHNICAL FIELD
[0002] The present disclosure relates generally to fuel cell
systems including one or more fuel cells, e.g. proton exchange
membrane fuel cells, engineered to have more than one internal
electrical resistance levels. The internal electrical resistance
level is engineered to change substantially with temperature. Such
changes to electrical resistance levels allow rapid heat-up of the
fuel cell system from low temperatures to an elevated temperature
that is optimal for water management and fuel cell operation. Such
fuel cell systems include hydrogen fuel cells for vehicle and
stationary applications.
BACKGROUND
[0003] Currently, fuel cells such as proton exchange membrane (PEM)
fuel cells have significant difficulties when starting in cold
environments, particularly at or below subzero temperatures. It is
widely believed that during PEM fuel cell operation at subzero
temperatures water produced from the oxygen reduction reaction
(ORR) forms ice in the cathode catalyst layer (CL) that hinders the
oxygen transport to the reaction sites, which can cause the PEM
fuel cell to eventually shut down. Several technologies attempt to
address operating fuel cells in cold environments.
[0004] For example, U.S. Pat. No. 6,358,638 B1 discloses an in-situ
chemical heating method to produce heat during cold start and hence
warm-up a fuel cell stack towards the freezing point. In this
method, either a small amount of O.sub.2 is injected into the anode
to induce O.sub.2--H.sub.2 combustion in the anode catalyst layer,
therebyby producing heat. Or a small amount of H.sub.2 is injected
into the cathode to induce H.sub.2--O.sub.2 combustion in the
cathode catalyst layer for heat production. In both cases, the
method is not effective as it also produces water which turns into
ice and fills up the catalyst layer so that the fuel cell becomes
inoperable. The amount of heat produced by this method is limited
by the water storage capacity of the catalyst layer and is rather
small due to the small void space in a thin catalyst layer.
Additionally, this method incurs degradation of the catalyst layer
as the H.sub.2--O.sub.2 catalytic reaction in the anode catalyst
layer will promote carbon corrosion in the cathode catalyst layer,
and H.sub.2--O.sub.2 catalytic reaction in the cathode catalyst
layer may result in hot spot formation over the membrane.
[0005] U.S. Pat. No. 8,263,278 B2 discloses an oxygen starvation
technique to maintain a low cell voltage and hence low-efficiency
operation such that there is more internal heat generated to warm
up a fuel cell stack. This method of oxygen starvation leads to
hydrogen pumping from the anode to cathode compartment, thereby
requiring dilution of the cathode exhaust in order to keep the
hydrogen concentration below a flammability limit before emitting
into the ambient. The oxygen starvation method also requires
elaborative control steps and may cause degradation of fuel cell
materials.
[0006] Therefore, it is desirable to develop a simple,
non-degrading method to rapidly start-up a fuel cell from subzero
temperatures.
[0007] Further, water management in fuel cells operating from low
ambient temperatures, e.g. room temperature, before reaching an
optimal range, e.g. 60-80.degree. C., has been exceedingly
difficult. Bulky humidification systems along with sophisticated
controls are employed in order to properly manage water and prevent
electrode, gas-diffusion layer and flow channel from flooding by
liquid water. A simple approach to this water management problem at
low temperatures could be a thermal method in which a cell is
quickly heated up from room-temperature to the design point of
elevated temperature. The higher cell temperature dramatically
promotes water evaporation and removal through vapor phase
diffusion, thereby alleviating flooding of fuel cell components and
materials.
SUMMARY OF THE DISCLOSURE
[0008] Advantages of the present disclosure include the design and
operation of fuel cell systems. The fuel cell systems of the
present disclosure are advantageously engineered to operate at
different internal electrical resistance levels based on a
temperature of one or more fuel cell in the system.
[0009] These and other advantages are satisfied, at least in part,
by a self-heating fuel cell system. The system comprises at least
one fuel cell and at least one resistor-switch unit which includes
a resistor and a switch wherein the switch is electrically
connected in parallel with the resistor. The switch can direct
current through the resistor (High Resistance State) or can direct
current to bypass the resistor (Low Resistance State). The resistor
is preferably placed in direct contact with the fuel cell so that
heat generated from the resistor can heat the fuel cell.
[0010] Embodiments of the present disclosure include any one or
more combinations of the following features. For example, the
system can include a stack of fuel cells wherein the at least one
fuel cell is among the stack of fuel cells. In addition, the one or
more fuel cells can individually include an anode catalyst
electrode, a cathode catalyst electrode and proton exchange
membrane therebetween. In some embodiments, the one or more fuel
cells can include an anode catalyst layer and/or a cathode catalyst
layer having ultrathin thicknesses, e.g., a thicknesses equal to or
less than 1 micrometer. In various embodiments, the one or more
fuel cells can further comprise a bipolar plate with a flowfield
for fuel flow on an anode, diffusion media on a membrane-electrode
assembly, a bipolar plate with a flowfield for oxidant flow on a
cathode, wherein the resistor is placed in direct contact with one
of the bipolar plates. In some embodiments the resistor is inside a
fuel cell, or outside of a fuel cell or when the system includes a
stack of fuel cells, the resistor-switch unit can be interposed
between two cells in the stack. In other embodiments, the resistor
can be a thin metal sheet having one or more electrically
insulating films on one or both major surfaces of the sheet and/or
the sheet can have tabs at either end of the resistor. In still
further embodiments, the system can include a controller for
operating the switch. In various embodiments, the controller can be
further configured to receive an input from a temperature sensing
device on the fuel cell. In some embodiments, the controller is
configured to operate the switch to an open state or a closed state
based upon an input from a temperature sensing device that can
measure a surface temperature of the at least one fuel cell.
[0011] Another aspect of the present disclosure includes a
self-heating fuel cell system having a first internal resistance
(R.sub.1) of the system when the temperature of the at least one
fuel cell is greater than a temperature (T.sub.1) and a second
internal resistance (R.sub.2) of the system when the temperature of
the at least one fuel cell is below T.sub.1, wherein the value of
R.sub.2 at about 2.degree. C. below T.sub.1 is at least twice the
value of R.sub.1 at T.sub.1. Such a self-heating fuel cell system
can include any one or more features described above and referenced
herein.
[0012] Further embodiments include any one or more combinations of
the following features. For example, wherein T.sub.1 is no greater
than 0.degree. C., e.g., between -5.degree. C. and 0.degree. C. In
other embodiments T.sub.1 is less than 95.degree. C., e.g., between
45 and 95.degree. C., and preferably between 60 to 80.degree.
C.
[0013] Another aspect of the present disclosure includes a method
of operating a self-heating fuel cell system. The method comprises
operating the system at a first internal resistance (R.sub.1) when
the temperature of at least one fuel cell in the system is greater
than a temperature (T.sub.1), and operating the system at a second
internal resistance (R.sub.2) of the system when the temperature of
the at least one fuel cell is below T.sub.1, by activating a switch
that activates either R.sub.1 or R.sub.2 depending on T.sub.1. The
self-heating fuel cell system can include any one or more of the
features described above or herein.
[0014] Additional advantages of the present disclosure will become
readily apparent to those skilled in this art from the following
detailed description, wherein only the preferred embodiment of the
disclosure is shown and described, simply by way of illustration of
the best mode contemplated of carrying out the disclosure. As will
be realized, the disclosure is capable of other and different
embodiments, and its several details are capable of modifications
in various obvious respects, all without departing from the
disclosure. Accordingly, the drawings and description are to be
regarded as illustrative in nature, and not as restrictive.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] Reference is made to the attached drawings, wherein elements
having the same reference numeral designations represent similar
elements throughout and wherein:
[0016] FIGS. 1A-1C schematically show characteristics of an
idealized self-heating fuel cell system where the resistance of a
fuel cell in the system changes according to temperature. FIG. 1A
shows a step jump at a threshold temperature T.sub.1; FIG. 1B shows
a sharp but smooth jump around a threshold temperature T.sub.1; and
FIG. 1C shows multi-step changes at multiple threshold
temperatures. The characteristic curves of a self-heating fuel cell
system of the present disclosure (drawn solid lines) are contrasted
to that of a conventional fuel cell (drawn in dashed lines).
[0017] FIGS. 2A-2D are a schematic illustration of a step-by-step
mechanism of how a self-heating fuel cell system according to an
embodiment of the present disclosure can promote internal heating
of a fuel cell from a subzero temperature. As shown in the figures:
a high internal resistance leads to low output voltage and high
internal heat generation which promotes a rapid rise in cell
temperature.
[0018] FIG. 3 is a schematic illustration of single fuel cell
having a resistor-switch unit (RSU) attached to a bipolar plate of
the fuel cell, according to an embodiment of the present
disclosure.
[0019] FIG. 4 illustrates the structure of another self-heating
fuel cell system with a resistor-switch unit interposed between two
fuel cells in a fuel cell stack, according to an embodiment of the
present disclosure.
[0020] FIG. 5 illustrates the structure of another self-heating
fuel cell system with multiple RSUs inserted between various fuel
cells in a fuel cell stack according to an embodiment of the
present disclosure.
[0021] FIG. 6 shows a self-heating fuel cell system with an RSU
inserted between the diffusion media and a bipolar plate according
to an embodiment of the present disclosure. The RSU is cut into the
same rib pattern as the bipolar plate.
[0022] FIG. 7 shows an embodiment of a thin-film resistor made from
a thin metal sheet covered on both major surfaces with insulating
films such as Kapton, and folded into a multi-layer compact
structure.
[0023] FIG. 8 shows one embodiment of a patterned resistor in the
form of a sheet and covered on both major faces by electrically
insulating films. The resistor is sandwiched between bipolar plates
of a lower and an upper fuel cell according to an embodiment of the
present disclosure. The two ends of the resistor are electrically
connected to the lower and upper bipolar plates, respectively,
along with a switch.
[0024] FIG. 9 is a diagram showing a fuel cell system according to
an embodiment of the present disclosure electrically connected to a
vehicle.
[0025] FIG. 10 is a chart showing current density and temperature
responses during the start of a fuel cell system from room
temperature according to an embodiment of the present
disclosure.
DETAILED DESCRIPTION OF THE DISCLOSURE
[0026] The present disclosure relates a self-heating fuel cell
system. Such a system can include at least one fuel cell and at
least one resistor-switch unit (RSU). The resistor-switch unit
includes a resistor and a switch wherein the switch is electrically
connected in parallel with the resistor. The switch can direct
current through the resistor (High Resistance State) or can direct
current to bypass the resistor (Low Resistance State). Electrical
current through the resistor generates heat. Preferably, the
resistor is placed in direct contact with the fuel cell so that
heat generated from the resistor can heat the fuel cell.
[0027] Advantageously, the change in internal resistance occurs at
a threshold temperature T.sub.1 which can be set according to the
particular fuel cell system but it typically less than 95.degree.
C. However, T.sub.1 can also be set to 0.degree. C. or less, e.g.,
between -5.degree. C. and 0.degree. C., so that the fuel cell can
self-heat rapidly from subzero temperatures. By this mechanism, a
fuel cell operation starting from room temperature or below will
exhibit a very high internal resistance level, which generates
immense heat and rapidly heats up the fuel cell to an optimal
operation range, e.g., around 60.degree. C. to 80.degree. C. Such a
structure can also facilitate water management in a PEM fuel cell,
for example.
[0028] In an aspect of the present disclosure, the fuel cell system
is configured to include a first internal resistance (R.sub.1) of
the system when the temperature of at least one fuel cell in the
fuel cell system is greater than a temperature (T.sub.1) and a
second internal resistance (R.sub.2) of the system when the
temperature of the of the at least one fuel cell is below T.sub.1.
Preferably the value of R.sub.2 changes abruptly, such as in a step
function, or changes sharply, such as in a continuous but rapid
change in resistance, around T.sub.1. For example, the value of
R.sub.2 at about 2.degree. C. below T.sub.1 is at least twice the
value of R.sub.1 at T.sub.1. Advantageously, the value of R.sub.2
at about 2.degree. C. below T.sub.1 is at least twice to fifty
times the value of R.sub.1 at T.sub.1. The change in internal
resistance of the fuel cell system is advantageously reversible,
i.e., the internal resistance can switch between R.sub.2 and
R.sub.1 around T.sub.1.
[0029] Advantageously, both the need for cold start and water
management of a fuel cell can be addressed by a self-heating fuel
cell system according to the present disclosure. For example, when
the cell temperature drops below a threshold value, e.g., below
about 0.degree. C., the cell's internal ohmic resistance can be
increased substantially so that the cell voltage and energy
efficiency become low, which in turn increases the internal heat
generation substantially. Furthermore, the ratio of heat to water
production can be significantly improved, which promotes rapid
heat-up of the fuel cell system from a subfreezing temperature
while significantly minimizing the possibility of fuel cell
shutdown by the product water that can turn into ice in a catalyst
layer. As used herein subfreezing and subzero mean a temperature
less than 0.degree. C.
[0030] The fuel cell system of the present disclosure can be
implemented in a variety of fuel cell configurations. For example,
the fuel cell system of the present disclosure can be implemented
in direct systems where fuel, such as hydrogen, methanol,
hydrazine, etc., can react directly in the fuel cell and indirect
systems in which fuel, such as natural gas or other fossil fuels,
is first converted by reforming to a hydrogen rich gas which is
then fed into the fuel cell. Such systems can be used to power
vehicles or stationary facilities, for example.
[0031] In some embodiments, the fuel cell system includes a proton
exchange membrane fuel cell that can operate on hydrogen or
hydrogen-containing gases. In other embodiments, the fuel cell
system is an alkaline membrane fuel cell, an
intermediate-temperature fuel cell operating between 100 and
300.degree. C., such as polybenzimidazole (PBI) fuel cells, or a
solid oxide fuel cell. All of these various fuel cell system
configurations can be used in the present disclosure.
[0032] In one aspect of the present disclosure, the fuel cell
system includes at least one fuel cell and at least one
resistor-switch unit. The resistor-switch unit is electrically
connected to the at least one fuel cell. Electrical current
generated by the fuel cell during operation of the system flows
through the resistor-switch unit and directly or indirectly to
another fuel cell or to an external load.
[0033] The major components of a fuel cell can include, for
example, an anode, cathode and electrolyte therebetween. In certain
configurations, the anode electrode includes an anodic catalyst
layer and a gas diffusion layer and the cathode electrode includes
a cathodic catalyst layer and a gas diffusion layer. The anode and
cathode can be sandwiched by bipolar plates having channels to
supply fuel to the anode and an oxidant to the cathode. Certain
fuel cells include a membrane-electrode assembly (MEA), such as
proton exchange membrane fuel cells, and with bipolar plates and
interconnects and sometimes gaskets for sealing/preventing leakage
of gases between the anode and cathode. Since the voltages
generated during the discharge of a single fuel cell tends to be
low, multiple fuel cells are preferably electrically connected
together to obtain higher voltages. Such systems are commonly
referred to as a fuel cell stack.
[0034] The resistor-switch unit of the self-heating fuel cell
system of the present disclosure includes a resistor and a switch,
wherein the switch is electrically connected in parallel with the
resistor. When the self-heating fuel cell system includes a fuel
cell stack, multiple resistor-switch units can be inserted between
fuel cells over several locations in the fuel cell stack. The
multiple resistor-switch units are electrically connected to the
fuel cells in the stack. In such systems, the multiple switches can
be activated sequentially according to a control algorithm.
[0035] Resistors that are useful for the present disclosure can be
made of, for example, graphite, highly ordered pyrolytic graphite
(HOPG), stainless steel, nickel, chrome, nichrome, copper,
aluminum, titanium, or combinations thereof. In certain
embodiments, the resistor of the present disclosure is preferably
flat with a large surface area so that it can have good thermal
contact with one or more fuel cells. In other embodiments, the
resistor can be a thin metal sheet having one or more electrically
insulating films on one or both major surfaces of the sheet. Such a
sheet can have tabs at either end of the resistor, which can be
used to electrically connect the resistor to one or more fuel cells
and to the switch. The resistor can have an overall thickness
between 1 and 100 micrometers.
[0036] In addition, the resistance of a resistor of the present
disclosure can be adjusted by forming a pattern within the resistor
such as forming a pattern within a resistor in the form of a sheet,
i.e., removing material from the resistor sheet. Patterning allows
a resistor to have a sufficient thickness for mechanical strength
and weldability but a large resistance. Such patterns include a
serpentine pattern, for example.
[0037] Electrically insulating films that can be used with the
resistors of the present disclosure include, for example,
polyethylene, polypropylene, chlorinated polypropylene, polyester,
polyimide, PVDF, PTFE, nylon, or co-polymers thereof. The
electrically insulating films can have a thickness between 0.1 and
50 micrometers.
[0038] The switch of the present disclosure can include those
activated by thermally sensitive devices such as a phase-change
material that undergoes phase transition and appreciable volume
change at T.sub.1, or a bimetal switch, or a solid material whose
volume expands appreciably at temperature T.sub.1, for example. In
addition, the switch of the present disclosure can be composed of
an electromechanical relay and a temperature controller, or a
solid-state relay with a temperature sensor, a power MOSFET with a
temperature sensor, or a high-current switch with a temperature
sensor.
[0039] In certain embodiments, the self-heating fuel cell system
can include a controller configured to operate the switch. The
controller can be further configured to receive an input from a
temperature sensing device, e.g., a thermocouple or a thermistor,
that can measure a surface temperature of the fuel cell. In some
embodiments, the controller can be configured to operate the switch
to an open state or a closed state based upon an input from the
temperature sensing device.
[0040] An advantage of the present disclosure is a fuel cell
system, such as a PEM fuel cell, whose internal resistance can
change according to cell temperature. When the cell temperature
lies below a threshold value, e.g. the freezing point, the cell's
ohmic resistance increases sharply such that the cell voltage and
efficiency become low, which increases the internal heat generation
substantially. Specifically, the heat generation rate of a fuel
cell can be related to the water production rate in a fuel cell as
follows:
q ( J / cm 2 ) = 2 F ( E h - V cell ) M H 2 O .DELTA. m H 2 O ( mg
/ cm 2 ) ##EQU00001##
where F is the Faraday constant, M.sub.H2O is the molecular weight,
E.sub.h is the thermodynamic cell potential (=.DELTA.h/2
F.about.1.48 V for H.sub.2--O.sub.2 reaction) and V.sub.cell is the
cell output voltage. See also Tajiri and Wang, Modeling and
Diagnostics of Polymer Electrolyte Fuel Cells, Vol 49, Chapter 3,
Cold Start of Polymer Electrolyte Fuel Cells, Springer Science,
2010. It is clearly seen that the ratio of heat generation to water
production is significantly higher with lower output voltage,
thereby promoting rapid warm-up of a fuel cell from a subfreezing
temperature while minimizing the possibility of clogging and
shutting down the fuel cell due to formation of water/ice in the
system.
[0041] An aspect of a self-heating fuel cell system of the present
disclosure is that the internal resistance of the system can change
abruptly when a fuel cell in the system reaches a threshold
temperature T.sub.1. Such changes in the internal resistance of a
self-heating fuel cell system are depicted in the idealized
resistance-to-temperature relationship in FIGS. 1A-1C, where a fuel
cell system of the present disclosure (solid lines) is contrasted
to that of a conventional fuel cell system (dashed lines). For
example and as shown in FIG. 1A, the internal resistance of a fuel
cell system of the present disclosure at a threshold value T.sub.1,
say between -5.degree. C. and 0.degree. C., can change as a step
function according to temperature. FIG. 1B illustrates a smooth,
but sharp change in the cell internal resistance around the
threshold temperature, say between -5.degree. C. and 0.degree. C.
FIG. 1C shows a multi-step change in the ohmic resistance of the
system at multiple threshold temperatures below T.sub.1, e.g., the
internal electrical resistance of the system below T.sub.1
(T.sub.1a and T.sub.1b) has multiple sub-levels (R.sub.2a and
R.sub.2b).
[0042] The mechanism by which a self-heating fuel cell system can
improve and accelerate startup of the system from a subfreezing
temperature is illustrated in FIGS. 2A-2D. For example, starting an
ambient temperature below the threshold value T.sub.1, the fuel
cell system can be operated in a high resistance state FIG. 2A,
resulting in low output voltage FIG. 2B and high internal heat
generation FIG. C. The latter leads to rapid rise in fuel cell
temperature FIG. 2D. Once the fuel cell temperature exceeds
T.sub.1, the system can be operated in a low resistance state
reducing the internal resistance of the system to as low as in a
conventional fuel cell system, making the electrical output voltage
and energy conversion efficiency as high as in a conventional fuel
cell.
[0043] FIG. 3 illustrates an embodiment of a self-heating fuel cell
system of the present disclosure. As shown in the figure, fuel cell
system 300 includes fuel cell 340. In this embodiment, fuel cell
340 is a membrane electrode assembly (MEA) with diffusion media
(315). The MEA with diffusion media includes anode catalyst layer
310 on diffusion medium 312, proton exchange membrane 320, and
cathode catalyst layer 330 on diffusion medium 332. A typical
example of a proton exchange membrane is one composed of a
perfluorosulfonic acid-tetrafluorethylene copolymer. The MEA is
sandwiched between two bipolar plates (314, 334) having channels to
supply fuel to the anode and an oxidant to the cathode (316 and
336, respectively). In this embodiment, bipolar plate 314 in which
gaseous fuel can flow (called the anode side), an anode diffusion
medium 312, a membrane electrode assembly, and a cathode diffusion
medium are connected to another bipolar plate 334 in which an
oxidant, e.g., air, can flow (called the cathode side). A
resistor-switch unit (RSU) can be attached to either bipolar plate.
In this embodiment, RSU 350 is electrically connected to the fuel
cell 340 through bipolar plate 334 and in direct contact with
bipolar plate 334.
[0044] As shown in the figure, RSU 350 includes upper sheet of
conductive metal 352 electrically connected and in direct contact
with bipolar plate 334, lower sheet of conductive metal 354,
thin-film resistor 356 interposed between upper sheet 352 and lower
sheet 354 and thermal switch 358 that can electrically connect
upper sheet 352 with lower sheet 354.
[0045] Current flowing through RSU is depicted in the blow-up
section of the figure. For example, when switch 358 is closed (Low
Resistance State), such as whenever the fuel cell temperature is
above a threshold value (T.sub.1), current predominately, if not
entirely, flows from the fuel cell through the sheets of conductive
metal (352 and 354) (shown as dotted lines (370)) effectively
bypassing thin film resistor 356 and thereby not adding extra
internal electrical resistance to the cell. However, when the cell
temperature drops below the threshold value T.sub.1, the switch is
opened (High Resistance State) which forces current generated from
the fuel cell to flow through the thin-film resistor (shown by
solid lines (380)) and thereby increasing the total cell resistance
of the system by AR and lowering the output voltage to a lower
level, e.g., the system output voltage can be lowered to about 0.1V
per cell. Joule heat generated on the resistor in turn accelerates
warm-up of the fuel cell to above T.sub.1. Once the cell
temperature rises above T.sub.1, the switch can be operated in the
closed state (Low Resistance State), and the internal resistance
recovers to a low value characteristic of a conventional fuel cell
without the RSU and the terminal voltage jumps to a normal
operation level under a certain current.
[0046] Although FIG. 3 illustrates one fuel cell and one RSU, the
fuel cell system of the present disclosure can have multiple fuel
cells such as a fuel cell stack and multiple RSU inserted between
fuel cells over several locations in the fuel cell stack.
[0047] Another embodiment of the present disclosure includes
fabricating a fuel cell system having at least one fuel cell and
RSU. Such a system can be made by a layer-by-layer deposition. For
example, a bipolar plate can be first used as a substrate to
deposit an electrically insulating layer, such as a poly(xylylene)
polymer (e.g., parylene), followed by depositing a conducting layer
(e.g., a metal) to form a serpentine-patterned resistor layer. The
conductive layer can be covered by another electrically insulating
layer, such as a poly(xylylene) polymer (e.g., parylene) coating.
Such a resistor layer can be electrically connected to two adjacent
bipolar plates. Such a structure can be useful for micro-fuel cell
systems as used in consumer electronics.
[0048] Additional embodiments of self-heating fuel cell system of
the disclosure are illustrated in FIGS. 4, 5 and 6. These
embodiments illustrate a resistor-switch unit (RSU) inserted
between two cells in a stack (FIG. 4), multiple RSUs interposed
between cells in a stack (FIG. 5), or a RSU inserted between a
bipolar plate and a diffusion medium (FIG. 6). In the latter case
(FIG. 6), the RSU is cut into a shape matching the rib pattern on
the bipolar plate.
[0049] For example, FIG. 4 shows RSU 450 between two fuel cells
(440a, 440b). The fuel cells include membrane electrode assemblies
(MEA) with diffusion media (415a and 415b, respectively) sandwiched
between two bipolar plates (414a, 434a and 414b, 434b) having
channels to supply fuel to an anode (416a, 416b) and an oxidant to
a cathode (436a, 436b).
[0050] In this embodiment, RSU 450 is electrically connected to the
fuel cells in the system through the bottom bipolar plate 434a of
fuel cell 440a and the top bipolar plate 414b of fuel cell 440b.
The RSU is in direct contact with both bipolar plates 434a and
414b. RSU 450 includes upper sheet of conductive metal 452
electrically connected and in direct contact with bipolar plate
434a, lower sheet of conductive metal 454 electrically connected
and in direct contact with bipolar plate 414b and thin-film
resistor 456 interposed between upper sheet 452 and lower sheet 454
and thermal switch 458 that can electrically connect upper sheet
452 with lower sheet 454.
[0051] RSU 450 can be operated in the same manner as described with
reference to FIG. 3. For example, when switch 458 is closed (Low
Resistance State), such as whenever the fuel cell temperature is
above a threshold value (T.sub.1), current predominately, if not
entirely, flows from the fuel cell through the sheets of conductive
metal (452 and 454) effectively bypassing thin film resistor 456
and thereby not adding extra internal electrical resistance to the
cell. However, when the cell temperature drops below the threshold
value T.sub.1, the switch is opened (High Resistance State) which
forces current generated from the fuel cells to flow through the
thin-film resistor and thereby increasing the total cell resistance
of the system by AR and lowering the output voltage to a lower
level. Joule heat generated on the resistor in turn accelerates
warm-up of the fuel cell to above T.sub.1. Once the cell
temperature rises above T.sub.1, the switch can be operated in the
closed state (Low Resistance State), and the internal resistance
recovers to a low value characteristic of a conventional fuel cell
without the RSU and the terminal voltage jumps to a normal
operation level under a certain current.
[0052] FIG. 5 shows another embodiment of a fuel cell system of the
present disclosure. As shown, two RSUs (550a, 550b) are located
between fuel cells (540a, 540b and 540b, 540c). The fuel cells
include membrane electrode assemblies (MEA) with diffusion media
(515a, 515b and 515c, respectively) sandwiched between bipolar
plates (534a, 514b, and 534b, 514c, respectively). The bipolar
plates have channels to supply fuel to an anode (516a, 516b, 516c)
and an oxidant to a cathode (536a, 536b, 536c).
[0053] In this embodiment, RSU 550a is electrically connected to
the fuel cells in the system through the bottom bipolar plate 534a
of fuel cell 540a and the top bipolar plate 514b of fuel cell 540b.
RSU 550b is electrically connected to the fuel cells in the system
through the bottom bipolar plate 534b of fuel cell 540b and the top
bipolar plate 514c of fuel cell 540c. The RSUs are in direct
contact with bipolar plates of adjacent fuel cells. Each of the
RSUs include an upper sheet of conductive metal electrically
connected and in direct contact with a bipolar plate, a lower sheet
of conductive metal electrically connected and in direct contact
with a bipolar plate and a thin-film resistor interposed between
the upper sheet and lower sheet and a switch that can electrically
connect the upper sheet with the lower sheet.
[0054] The RSUs of this embodiment can be operated in the same
manner as described with reference to FIG. 4. For example, when the
switch is closed (Low Resistance State), such as whenever the fuel
cell temperature is above a threshold value (T.sub.1), current
predominately, if not entirely, flows from the fuel cell through
the sheets of conductive metal effectively bypassing thin film
resistor. However, when the cell temperature drops below the
threshold value T.sub.1, the switch is opened (High Resistance
State) which forces current generated from the fuel cells to flow
through the thin-film resistor and thereby increasing the total
cell resistance of the system by .DELTA.R and lowering the output
voltage to a lower level. Joule heat generated on the resistor in
turn accelerates warm-up of the fuel cell to above T.sub.1. Once
the cell temperature rises above T.sub.1, the switch can be
operated in the closed state (Low Resistance State), and the
internal resistance recovers to a low value characteristic of a
conventional fuel cell without the RSU and the terminal voltage
jumps to a normal operation level under a certain current.
[0055] FIG. 6 shows an embodiment of the present disclosure where a
RSU is inserted between a bipolar plate and a diffusion medium. As
shown in the figure, fuel cell 640 includes a membrane electrode
assembly (MEA) with diffusion media (615). The MEA with diffusion
media includes a anode catalyst layer on diffusion medium, proton
exchange membrane, and a cathode catalyst layer on diffusion
medium, which are not shown for illustrative convenience. The MEA
is sandwiched between two bipolar plates (614, 634) having channels
to supply fuel to the anode and an oxidant to the cathode (616 and
636, respectively).
[0056] In this embodiment, RSU 650 is inserted between bipolar
plate 634 and a diffusion medium of fuel cell 640. The diffusion
medium can be that associated with the cathode or anode. RSU 650
includes upper sheet of conductive metal 652 electrically connected
and in direct contact with the diffusion medium, lower sheet of
conductive metal 654, thin-film resistor 656 interposed between
upper sheet 652 and lower sheet 654 and thermal switch 658 that can
electrically connect upper sheet 652 with lower sheet 654. As shown
in the A-A view, lower sheet 654 has a serpentine pattern which is
to match the serpentine pattern of bipolar plate 634.
[0057] For example, when switch 658 is closed (Low Resistance
State), such as whenever the fuel cell temperature is above a
threshold value (T.sub.1), current predominately, if not entirely,
flows from the fuel cell through the sheets of conductive metal
(652 and 654) effectively bypassing thin film resistor 656.
However, when the cell temperature drops below the threshold value
T.sub.1, the switch is opened (High Resistance State) which forces
current generated from the fuel cell to flow through the thin-film
resistor and thereby increasing the total cell resistance of the
system by .DELTA.R and lowering the output voltage to a lower
level. Joule heat generated on the resistor in turn accelerates
warm-up of the fuel cell to above T.sub.1. Once the cell
temperature rises above T.sub.1, the switch can be operated in the
closed state (Low Resistance State), and the internal resistance
recovers to a low value characteristic of a conventional fuel cell
without the RSU and the terminal voltage jumps to a normal
operation level under a certain current.
[0058] In another aspect of the present disclosure, the resistor in
a RSU can be made from a thin metal sheet such as 25 um stainless
steel sheet and covered by an insulating film, e.g., a 7.5 um
Kapton film on both major surfaces. Such a resistor can be folded
into a compact multi-layer structure and used in an RSU. FIG. 7
illustrates such an RSU. As shown in the figure, thin metal sheet
710 is coated with an insulating layer 712 on both major surfaces
thereof 712a and 712b. Thin metal sheet 710 has exposed areas
(e.g., areas without the insulating film) for forming electrical
contacts (710a and 710b). Thin metal sheet 710 can be folded into
compact multi-layer structure 714. The multilayer structure can be
inserted between adjacent fuel cells (740a, 740b) such that exposed
area 710a is in electrical connection with a bottom bipolar plate
of one cell and exposed 710b is in electrical contact with a top
bipolar plate of an adjacent cell. Switch 758 can electrically
connect the bipolar plates in electrical contact with the
multi-layer structure 714 to bypass the multilayer structure
(closed state) or to force current through the multi-layer
structure (open state).
[0059] The uncoated metal surfaces at the top and bottom of the
folded structure are electrically connected to two bipolar plates
which acts similar to the conductive sheets depicted in FIG. 3. The
configuration of the two bipolar plates, folded resistor and switch
shown in FIG. 7 operates as the RSU described in FIG. 3.
[0060] If the presence of the resistor is designed to lower the
fuel cell output voltage from normally around 0.6V/cell to around
0.1V/cell, the resistance of the resistor can be roughly estimated
to be 0.5V/I, where I is the current generated from the fuel cell.
For example, if a fuel cell generates 200 amps, the resistance of
the resistor should be designed as 2.5 m.OMEGA./cell.
[0061] Another aspect of the present disclosure is a multi-step
change in the internal electrical resistance of a self-heating fuel
cell system at multiple threshold temperatures below T.sub.1. Such
a profile can provide appropriate power adjustments when the system
is below normal operating temperatures. For example and as
illustrated in FIG. 5, switch #2 (550b) can be set to close (Low
Resistance State) at a threshold temperature (T.sub.1a), say
-15.degree. C., while keeping other switches still open (High
Resistance State). This will provide some power boost from
-15.degree. C. and upwards. Subsequently, switch #1 (550a) can be
set to close (Low Resistance State) at T.sub.1, e.g., -5.degree.
C., giving another power boost. One can design and execute a
control algorithm to turn on various switches according to fuel
cell temperatures such that power output can be appropriately
modulated.
[0062] FIG. 8 shows another embodiment of the present disclosure.
Here, the bipolar plate between an upper and a lower fuel cell is
separated into two parts with a RSU in between. For example, the
figure shows bipolar plate 834 with a serpentine flowfield 820 and
resistor sheet 856 having insulating films on both major surfaces
thereof embedded in plate 834. Bipolar plate 834 would be between
two fuel cells, e.g., upper part of bipolar plate 834a would
contact an upper fuel cell and lower part of bipolar plate 834b
would contact a lower fuel cell, which are not shown for
illustrative convenience.
[0063] In this embodiment, resistor 856 in the RSU is a patterned
electrically conductive thin sheet with two tabs at the two ends
(856a, 856b). The tabs electrically connect resistor sheet to the
upper and lower portion of the bipolar plate. Tab 856a electrically
connects resistor sheet 856 to upper portion of bipolar plate
(834a) which is electrically connected to an upper fuel cell and
tab 856b electrically connects resistor sheet 856 to lower portion
of bipolar plate (834b) which is electrically connected to a lower
fuel cell. Both major faces of the sheet are covered or coated by
electrically insulating films. Examples of insulating materials
include: polyethylene, polypropylene, chlorinated polypropylene,
polyester, polyimide, PVDF, PTFE, nylon, or co-polymers of them.
The two tabs of the sheet are electrically connected to the lower
and upper parts of the bipolar plate, respectively. Additionally,
switch 858, which can be controlled by a temperature controller, is
electrically connected to the two tabs (856a, 856b) of resistor
sheet 856. When the cell temperature is below the threshold point
T.sub.1, the switch is made OPEN, thus adding an extra resistance
from the resistor sheet to the fuel cell. Once the cell temperature
rises beyond T.sub.1, the switch is CLOSED, making the current
bypass the resistor sheet and hence resuming the fuel cell internal
resistance to a low level.
[0064] FIG. 9 illustrates a self-heating fuel cell system of the
present disclosure configured to operate a motor vehicle. As shown
in the figure, the fuel cell system of this embodiment includes
fuel cell 10 having resistor 20 electrically connected and in
direct contact thereto and switch 30. When switch 30 is open as
shown in the figure, current flows through resistor 20 generating
heat which heats fuel cell 10. The system further includes DC/DC
converter 130 electrically connected to fuel cell 10 and inverter
100, electric motor 110 electrically connected to inverter 100 and
controller 80 electrically connected to switch and configured to
open the switch to generated heat in the resistor to heat the fuel
cell. Electric motor 110 can be mechanically connected to a drive
train to drive wheels 120L and 120R of the vehicle.
[0065] The system can also include voltage sensor 150, current
sensor 140 and temperature sensor 50 which are all electrically
connected to fuel cell 10 and electrically connected to controller
80 and which can all provide inputs to controller 80.
[0066] In operation and on receiving a starting command of the
system, controller 80 detects a surface temperature of the fuel
cell 10 using temperature sensor 50 or an equivalent device. The
controller 80 determines that warm-up operation is needed in cases
where the surface temperature of fuel cell 10 is below a preset
threshold temperature, and shifts switch 30 off. The resistor 20 is
operated in the circuit and the fuel cell system is operated at the
high resistance mode, lowering the terminal voltage. The controller
simultaneously requests DC/DC converter 130 to convert low terminal
voltage 150 and certain current 140 into a sufficiently
high-voltage power for inverter 100. The inverter then drives
electrical motor 110 which is connected to wheels 120. When the
temperature of fuel cell 10 reaches the threshold temperature,
controller commands switch 30 to on to bypass resistor 20.
[0067] The present disclosure is especially effective for fuel
cells with untrathin catalyst layers, such as 1 micrometer and
less, in which water management at low temperatures is known to be
very difficult. The present disclosure also applies to other types
of fuel cells and their systems, such as alkaline membrane fuel
cells, intermediate temperature fuel cells such as phosphoric acid
fuel cells, and solid oxide fuel cells, for rapid startup.
EXAMPLE
[0068] The following examples are intended to further illustrate
certain preferred embodiments of the present disclosure and are not
limiting in nature. Those skilled in the art will recognize, or be
able to ascertain, using no more than routine experimentation,
numerous equivalents to the specific substances and procedures
described herein.
[0069] A PEM fuel cell of 25 cm.sup.2 was assembled using
stamped-metal bipolar plates for both anode and cathode sides. A
serpentine flow field of dimensions 0.6 mm width and 0.35 mm height
with the rib width of 0.6 mm was used. The MEA used in this cell
included 18 um fluorinated composite membrane and catalyst layers
with 0.32 mg/cm.sup.2 Pt-loading on each side. Teflon-coated
Toray-030 carbon paper coated with 30 um microporous layer (MPL)
was used as the gas diffusion media. The resistor in the form of a
sheet was attached to the cathode plate and sized to have a
resistance of 0.2 Ohm or 0.5 Ohm-cm.sup.2. The thermal mass of the
entire cell was estimated to be 0.1 J/(cm.sup.2K). The cell was
sandwiched between two Lexan plates to provide adiabatic
conditions.
[0070] The first set of tests was run to self-start the fuel cell
from room temperature, i.e. 20.degree. C., and at pressure of 2 atm
(absolute) on both fuel and air sides. The hydrogen and air flows
were at 0 and 60% relative humidity corresponding to the ambient
temperature with flow stoichiometries of 1.5 and 2 at 1 A/cm.sup.2,
respectively. The resulting current and temperature versus time are
shown in FIG. 10 for two cases, one with the cell voltage kept
constant at 0.1V and the other at 0.2V. It is seen that the cell
warms up from room temperature to an optimal operating temperature
of 60.degree. C. in just 2.7 seconds in the case of 0.1V and 3.4
seconds at 0.2V, respectively. In the meanwhile, the current
density increases from .about.0.9 at t=0 to 1.2 A/cm.sup.2 when the
cell temperature reaches 60.degree. C. for the 0.1V case, and from
0.75 to 1.02 A/cm.sup.2 in the 0.2V case. These tests clearly
demonstrate that a fuel cell system including a fuel cell and the
resistor according to present disclosure is able to rapidly warm-up
by itself, leaving fuel cell operation in the suboptimal range only
lasting for a few seconds under any ambient conditions. Therefore,
the self-heating fuel cell systems of the present disclosure can
greatly improve water management of the fuel cell at low
temperatures compared to conventional fuel cell systems. It is even
possible to completely eliminate any humidification systems for a
PEM fuel cell system, as shown in the above-described
experiments.
[0071] Only the preferred embodiment of the present disclosure and
examples of its versatility are shown and described in the present
disclosure. It is to be understood that the present disclosure is
capable of use in various other combinations and environments and
is capable of changes or modifications within the scope of the
inventive concept as expressed herein. Thus, for example, those
skilled in the art will recognize, or be able to ascertain, using
no more than routine experimentation, numerous equivalents to the
specific substances, procedures and arrangements described herein.
Such equivalents are considered to be within the scope of this
disclosure, and are covered by the following claims.
* * * * *